[SI-LIST] : Decoupling capacitor selection & placement

Gopala Krishna M.R. (gkrishna@rri.ernet.in)
Wed, 29 Oct 1997 12:11:52 +0530 (IST)

Hello Andrew, and everyone at SI-List!

About decoupling capacitor (D-CAP) placement:

One can place it as much as possible nearer to the component.
About the number of D-CAPs it is better to follow the Manufacturer's
data sheets. Need not worry about placing a D-CAP just near to the
power pin itself. Decoupling will be effective if the required
D-CAPs are placed around the specific component. I feel that we should
not reduce the number of D-CAPs per component below the data sheet number,
that may cause the problem with de-coupling. This technique is working
fine with us.

Any modification and questions about this are welcome.

Regards,

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Gopala Krishna M R
(email : gkrishna@rri.ernet.in)
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On Wed, 29 Oct 1997, Andrew Phillips wrote:

> Hello,
>
> Please forgive me if my questions have been asked a million times before
> ...
>
> When using decoupling capacitors to provide a low-impedance path between
> power and ground it appears to be well-established that we must do the
> following:
>
> - from device Vcc pin we drop a via to Vcc plane with as short a
> connection as possible (to minimize lead inductance). Same for device
> Gnd pin.
> - from capacitor pads we also drop vias to Vcc and Gnd planes with very
> short connections.
>
> What is the ruling for how close the capacitor needs to be to the device
> Vcc and Gnd pins?
>
> For a device such as a microprocessor with multiple Vcc and Gnd pins,
> how do we determine how many capacitors will be required?
>
> Many references suggest that using 1 x 0.1 uF cap per Vcc-Gnd pair is a
> good rule-of-thumb - how is this value determined, and are there
> situations when it is invalid?
>
> Thanks for any help,
>
> Andrew Phillips
> Cooperative Research Centre for Broadband Telecommunications &
> Networking
> Perth, Western Australia
>
> p.s. anyone interested in the Texas Instruments TMS320C6x DSP, please
> check out my info site:
>
> http://www.atri.curtin.edu.au/~andrew/DSP_Design/tms320c6x.html
>